Allow custom notifications

[tsmaeder]

While jdt.ls extenders can add functionality by adding custom commands, there is not such ability for notifications. As it happens, The remote proxy we get back from Launcher.getRemoteProxy() when starting the client connection in JavaLanguageServerPlugin#startConnection() implements "Endpoint". This would allow us to add a method to JDTLanguageServer like so:

public void notify(String method, Object parameter) {
  ((Endpoint)client).notify(method, parameter);
}

This would very simply allow custom notifications.

[fbricon]

Superseded by #595